Tcp sequencing and IP fragmentation

Posted on 2004-09-06
Last Modified: 2013-11-29
I know TCP segments are numbered (with sequence numbers) for reassembly/error recovery reasons.  
How do TCP segments and IP fragmentation correlate?  If the sender has an IP datagram that exceeds the layer 2's MTU, it fragments the datagrams and each datagram gets sent as separate frames across the wire.

Now, what is responsible for reassembly on the other end?   I know that with packet fragmentation, a packet id is defined and the "fragment" flag is turned on.  What is TCP's job in the reassembly of fragged IP datagrams?  Arent the fragmented packets ID'd anyway?
Question by:dissolved
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
  • 3

Accepted Solution

LimeSMJ earned 500 total points
ID: 11991697
The Internet layer (IP protocol) within the TCP/IP architecture is responsible for reassmebly of a fragmented packet.

If a packet hits a network where the MTU is lower than the originator, the packet is fragmented to allow for transmission.  The fragmentation is done usually by the routers along the way so the sender does not know of the packet manipulation (sender won't decrease packet size if MTU is smaller).  Since the sender is only sending sequenced packet numbers (not caring about fragmentation), the fragmented packets need to have their own sequencing flags added in by the router - so that the IP layer can defragment at the receiving end.

TCP's job is provide the transmission link... IP helps negotiate the data packets along the way.


Author Comment

ID: 11991843
Cant fragmentation occur between the network layer and data link layer as well?
I heard that if an IP datagram is handed to the data link layer, it will be fragmented if it is to big. And each packet leaving the network will have the fragment flag turned on.


Author Comment

ID: 11991850
Also, cant the receiver send a ICMP error back to the sender saying "needs fragmentation" or something like that?
Efficient way to get backups off site to Azure

This user guide provides instructions on how to deploy and configure both a StoneFly Scale Out NAS Enterprise Cloud Drive virtual machine and Veeam Cloud Connect in the Microsoft Azure Cloud.


Expert Comment

ID: 11992029
The ICMP request for fragmentation is generated by a router if the packet received has the "do not fragment" bit set and cannot be forwarded because the next hop MTU is too small for the packet (fragmenting is not possible to make the MTU smaller, so this error occurs).  The actual destination computer would never send an ICMP request for fragmentation.

Expert Comment

ID: 11992101
I am not sure what you meant by :

"...IP datagram is handed to the data link layer, it will be fragmented if it is to big..."

The IP datagram is the packet itself... the sent payload.  If the sending computer knows of potential MTU problems, it may allow the packet to be fragmented (thus preventing any transmission delays).  Unless there is a stack misconfiguration, there is no way for a datagram to be internally fragmented between the OSI layers.

Author Comment

ID: 11992105

Featured Post

Manage your data center from practically anywhere

The KN8164V features HD resolution of 1920 x 1200, FIPS 140-2 with level 1 security standards and virtual media transmissions at twice the speed. Built for reliability, the KN series provides local console and remote over IP access, ensuring 24/7 availability to all servers.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Title # Comments Views Activity
Setting out for Cisco UCS 2 66
Looking for a program called HoneyMine. 3 70
Distinguished username as email address 4 45
Blocking outside IP Addresses 16 55
Let’s list some of the technologies that enable smooth teleworking. 
Meet the world's only “Transparent Cloud™” from Superb Internet Corporation. Now, you can experience firsthand a cloud platform that consistently outperforms Amazon Web Services (AWS), IBM’s Softlayer, and Microsoft’s Azure when it comes to CPU and …
After creating this article (, I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…
Get a first impression of how PRTG looks and learn how it works.   This video is a short introduction to PRTG, as an initial overview or as a quick start for new PRTG users.

733 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question